The present invention relates to a method for detecting decrease of tire air pressure and an apparatus used therefor. More particularly, the invention relates to a method for detecting decrease of tire air pressure regardless of whether a vehicle is running at a low velocity or at a high velocity and an apparatus used therefor.
In recent years, as one of the safety devices for the 4-wheel vehicles such as passenger cars and trucks, there have been proposed devices for detecting decrease of tire air pressure, some of which have already been in practical application.
The devices for detecting decrease of tire air pressure are those developed under recognition of the importance thereof mainly for the following reasons. Namely, when the air pressure decreases, the temperature of the tire increases due to increase of deflection. When the temperature increases, the strength of polymer materials used for the tire decreases, leading to bursting of the tire. Normally, it frequently occurs for the driver not becoming aware of the loss of air in the tire by about 0.5 atm. For this reason, an apparatus capable of detecting such loss of air pressure has been desired.
The method of detecting decrease of air pressure in the above-mentioned tire air pressure detecting device includes, for example, one based on the difference of the angular velocity of rotation F1, F2, F3 and F4 (hereinafter, when generally called, referred to angular velocity of rotation Fi) of four tires W1, W2, W3 and W4 provided on the vehicle (tires W1 and W2 correspond to the front left and right tires, and tires W3 and W4 correspond to the rear left and right tires, respectively; hereinafter, when generally called, referred to tire Wi).
According to the method, for example, based on signals outputted from the wheel velocity sensor fitted to the tire Wi, the angular velocity of rotation Fi of the tire Wi is detected on each predetermined sampling period. Here, the detected angular velocity of rotation Fi is all the same, provided that the effective rolling radii of tire Wi (amount obtained by dividing the distance of vehicle advance by 2.pi. when the tire rotates by one turn during free rotation of tire) are all the same, and the vehicle runs linearly.
On the other hand, the effective rolling radii of tire Wi often vary due to the variation of air pressure of the tire Wi. Namely, when the air pressure of the tire Wi decreases, the effective rolling radius becomes smaller than in the case of normal inner pressure. Accordingly, the angular velocity of rotation Fi of the tire Wi in which its air pressure is lowered becomes faster than in the case of normal inner pressure. Therefore, the decrease of air pressure of the tire Wi can be detected by the difference of each angular velocity of rotation Fi.
The judgement equation for detecting decrease of air pressure of the tire Wi by the difference of angular velocity of rotation Fi is, for example, one as shown in the following equation (1) (refer to, for example, Japanese Unexamined Patent Publications No. 305011/1988 and No. 212609/1992). EQU (F.sub.1 +F.sub.4 -F.sub.2 -F.sub.3)/{(F.sub.1 +F.sub.2 +F.sub.4)/2}(1)
Assuming that the effective rolling radii of the above tires Wi are all the same, the angular velocity of rotation Fi becomes all the same (F.sub.1 =F.sub.2 =F.sub.3 =F.sub.4), so that the judgement value D is 0. Therefore, when the threshold value D.sub.TH1 and D.sub.TH2 are set and the following equation (2) is satisfied, it is judged that there is a tire Wi in which the air pressure is lowered, and when the equation (2) is not satisfied, it is judged that there is no tire Wi whose air pressure is lowered. Further, when it is judged that there is a tire Wi whose air pressure is lowered, an alarm is issued by, for example, a display device. EQU D&lt;-D.sub.TH1 or D&gt;D.sub.TH2 ( 2)
However, with only the judgement of the air pressure decrease based on the above equations (1) and (2), there is a drawback of possibility of erroneous detection, depending on the vehicle running condition. For example, when the air pressure of any one tire Wi of the tires Wi shows decrease in air pressure, accurate detection can be made whether the air pressure is lowered or not if the vehicle is running at a relatively low velocity, while if the vehicle is running at a relatively high velocity with being driven, there is a possibility of erroneous detection to be made. The reason is because the slip rate of the depressurized wheel during the high velocity running is lowered, and the rolling radius increases by the centrifugal force of the tire.
In view of the above circumstances described above, an object of the present invention is to provide a method for detecting decrease of tire air pressure and an apparatus used therefor capable of detecting accurately decrease of tire air pressure regardless of the vehicle running condition, thereby preventing erroneous issuance or non-issuance of alarm (that is, notwithstanding the pressure decrease, no alarm is activated).
A further object of the present invention is to provide a method for detecting decrease of tire air pressure and an apparatus used therefor capable of detecting accurately decrease of tire air pressure regardless of the vehicle running condition, especially regardless of the vehicle running velocity, thereby preventing erroneous issuance or non-issuance of alarm.